1. Field of the Invention
The invention relates to an air-fuel ratio control system of an internal combustion engine, which performs air-fuel ratio feedback control through main feedback control based on the concentration of oxygen in exhaust gas upstream of a catalyst, and sub feedback control based on the concentration of oxygen in exhaust gas downstream of the catalyst.
2. Description of the Related Art
In the internal combustion engine installed on a vehicle, for example, a catalyst provided in an exhaust passage cleans up harmful components in exhaust gas. The catalyst's ability to purify exhaust gas varies depending on the air-fuel ratio as the ratio of air to fuel in an air-fuel mixture burned in a combustion chamber, and the range of the air-fuel ratio within which the catalyst exhibits the maximum exhaust purifying ability is limited. In the case of a three-way catalyst, for example, when the air-fuel ratio of the air-fuel mixture is in the vicinity of the stoichiometric air-fuel ratio at which the entire amount of oxygen in the mixture is theoretically used for combustion of the fuel, the catalyst exhibits the maximum exhaust purifying ability. In many internal combustion engines installed on vehicles, for example, the air-fuel ratio of the mixture burned is obtained from the concentration of oxygen remaining in exhaust gas, and the fuel injection amount is corrected in accordance, with the obtained air-fuel ratio, so that air-fuel ratio feedback control for making the air-fuel ratio of the mixture equal to the optimum value for purification of exhaust gas is performed.
If an oxygen concentration sensor for measuring the concentration of oxygen in exhaust gas is able to show ideal output characteristics, the output of the sensor has a unique relationship with the air-fuel ratio of the air-fuel mixture burned. In this case, if the target air-fuel ratio is set so as to be within a catalyst window as a air-fuel ratio suitable far purification of exhaust gas by the catalyst, and feedback control is executed so that the air-fuel ratio calculated from the measured oxygen concentration of exhaust gas becomes equal to the target air-fuel ratio, only the exhaust gas that has been purified flows out of the catalyst, into the exhaust passage downstream of the catalyst.
However, in reality, the oxygen concentration sensor may not necessarily exhibit ideal output characteristics, due to individual differences and chronological changes in the oxygen concentration sensor and its signal transmission system, changes in the operating conditions of the internal combustion engine, etc. Therefore, even under a situation where the air-fuel ratio feedback control is executed, unpurified exhaust gas may flow downstream of the catalyst. In this case, the oxygen concentration of exhaust gas downstream of the catalyst deviates from a value corresponding to the stoichiometric air-fuel ratio established when the amount of oxygen in exhaust gas is not excessive nor insufficient for exhaust purifying reactions. Therefore, it can be determined from the measurement result of the concentration of oxygen in exhaust gas downstream of the catalyst whether exhaust gas is appropriately purified in the catalyst. Thus, more accurate air-fuel ratio control is realized by performing sub feedback control based on the oxygen concentration of exhaust gas downstream of the catalyst, in addition to main feedback control based on the oxygen concentration of exhaust gas upstream of the catalyst.
A certain length of time is required for the result of the sub feedback control to converge on the optimum value. Therefore, a constant component of a sub feedback correction value obtained in the past sub feedback control is stored as a sub feedback learned value, and the learned value is reflected by the sub feedback control, in an attempt to reduce the time required for convergence.
In some types of internal combustion engines installed on vehicles, exhaust gas recirculation (EGR) for recirculating a part of exhaust gas into intake air is conducted. In the internal combustion engine having the EGR function, it is found that the catalyst window changes when the EGR is carried out. Thus, an air-fuel ratio control system that sets a feedback target value (target air-fuel ratio) of air-fuel ratio feedback control to values that are different depending on whether or not the EGR is conducted has been proposed, as described in Japanese Patent Application Publication No. 2005-030339 (JP 2005-030339 A). This type of air-fuel ratio control system changes the air-fuel ratio of the air-fuel mixture in accordance with change of the catalyst window depending on whether or not the EGR is conducted. Thus, the control system provides advantageous effects, such as reduction of NOx and improvement of the fuel efficiency, owing to the EGR, while assuring desired exhaust purifying efficiency of the catalyst.
In the air-fuel ratio control system of the internal combustion engine which performs sub feedback control as described above, if the feedback target value (target air-fuel ratio) of the main feedback control is set to values that are different depending on whether or not the EGR is conducted, it is possible to achieve advantageous effects, such as reduction of NOx and improvement of the fuel efficiency, owing to the EGR, while assuring desired exhaust purifying efficiency of the catalyst.
If an EGR valve is stuck in a closed position, namely if the EGR valve is held closed and will not open, the amount of exhaust gas recirculated into the intake air during execution of the EGR may be reduced to be smaller than the originally required amount. In this case, the amount of change in the catalyst window due to the EGR is smaller than that in the case where the EGR is normally conducted.
In this case, the target air-fuel ratio of the main feedback control is set on the assumption that the EGR amount as required is ensured; therefore, the target air-fuel ratio is set to a value that deviates from the actual catalyst window. The deviation of the target air-fuel ratio from the actual catalyst window is compensated for through sub feedback control. Therefore, if the air-fuel ratio feedback control is continued in this condition, the deviation is taken into the sub feedback learned value. Namely, the sub feedback learned value obtained at this time deviates from a proper value by an amount corresponding to a difference between the catalyst window in the case where the EGR is normally conducted and the catalyst window in the case where the EGR is not normally conducted. Therefore, if the EGR is subsequently stopped, and the feedback target value of the main feedback control is changed to the value to be achieved when the EGR is not conducted, a shift may occur to the air-fuel ratio under an influence of the deviation of the sub feedback learned value as described above. In this case, emissions (or exhaust characteristics) of the internal combustion engine may deteriorate until the air-fuel ratio feedback control settles.